The fierce urgency of now
I think you've put your finger on one of the essential differences between the media narratives we've been discussing: the imperative that Something Must Be Done, coupled with politically packaged suggestions for what that Something should be.
Certainly people who live in urban war zones know the crime is happening, although I wonder how numb the average local reader of the Chicago Tribune has grown to the weekly death toll. People who don't live there have a general impression that things are pretty bad, although as we saw in Oklahoma, they're shocked and horrified to find gang violence rolling into their communities. Gang culture is spreading faster and further than anyone thought it would, twenty or thirty years ago.
The gang violence we're talking about is reported with a sad "what are you gonna do?" air of resignation. Somehow the bumbling mayor of Chicago survived a public appeal to the "values" of gang bangers, asking them to aim more carefully so they don't kill as many innocent civilians in the crossfire. In an earlier, better age, in cities other than Chicago, he'd have been run out of town on a rail. But he's got all the right connections and support from the Democrat machine, so he gets to stand knee-deep in pools of blood and call for everyone else to adopt the policies of his failed city government. No one really expects anyone to do anything decisive about the violence, or explain why all those gun-control nostrums haven't helped.
But look at how the Zimmerman-Martin story was immediately packaged with calls to action. From the first moment of national exposure for the story, there were coordinated protests demanding George Zimmerman's arrest, based on the entirely false media narrative that the local cops were a pack of incompetent racists for letting him go. (They were, in truth, merely following Florida law for credible claims of self-defense. It took a lot of media truth-sculpture to obscure that fact, and make it look like Zimmerman's claims were not credible. This effort was helped by the actions of the thoroughly intimidated, panic-stricken local government.)
Now we've moved right along to the howls for another Zimmerman trial to procure the correct "guilty" verdict, coupled with specific legislative remedies, which people who still think Trayvon Martin was "murdered" are encouraged to support, in order to "fix" the problem of racist homeowners hunting down black kids. As you put it, "presto chango" - real problems are swept aside, including the real problem of Trayvon Martin's behavior on that fateful night, and his troubled personal history. We are angrily instructed to forget about those topics. Pay no attention to what lurks behind the curtain. It's a shabby magic trick, which unfortunately requires a lot of audience participation to pull off.